Tuning laser-induced optical breakdown and cavitation through the ionic environment in aqueous media

TL;DR

The study addresses how the ionic environment in water modulates laser-induced optical breakdown and cavitation. It decouples ionic strength from ion-specific chemistry by studying neutral salt solutions and strongly acidic/alkaline systems using nanosecond 1064 nm pulses, measuring cavitation thresholds $E_c$, bubble counts $N_c$, cavitation-zone length $L_c$, and conductivity. The results show that higher ionic strength lowers the breakdown threshold by enhancing seed-electron availability, but ion identity matters: $H^+$ can quench hydrated electrons and raise $E_c$, while $OH^-$ can remove the hydronium scavenger and lower $E_c$, promoting more intense cavitation. Importantly, electrical conductivity remains nearly constant across pH, indicating that microscopic hydrated-electron chemistry, not bulk charge transport, governs cavitation onset. These findings connect electrolyte chemistry with laser–plasma processes and offer a route to tune plasma generation and cavitation in chemically complex fluids for laser processing, sonochemistry, and biomedical applications.

Abstract

Laser-induced cavitation in liquids originates from optical breakdown processes that depend sensitively on both laser-plasma dynamics and the chemical microenvironment of the solvent. Herein, we experimentally decouple the effects of ionic strength and ion specificity on cavitation inception in aqueous electrolytes spanning neutral, acidic, and alkaline regimes. Using focused nanosecond laser pulses, we show that increasing ionic strength universally lowers the cavitation threshold by enhancing charge screening and seed-electron availability. However, under constant ionic strength, strongly asymmetric behavior emerges: acidic (hydrogen chloride, HCl) solutions inhibit cavitation, whereas alkaline (sodium hydroxide, NaOH) solutions enhance it. This asymmetry arises from hydrated-electron kinetics that depends on the ion specificity. In acidic solutions, hydronium ions act as diffusion-limited scavengers of hydrated electrons, quenching their lifetime and inhibiting avalanche ionization. In contrast, hydroxide ions reduce hydronium availability and extend electron survival, promoting more efficient plasma formation. Despite large discrepancies in breakdown thresholds, electrical conductivity remains nearly constant, demonstrating that microscopic electron chemistry, rather than bulk charge transport, governs cavitation onset. These results establish a mechanism connection between electrolyte chemistry and optical breakdown, showing that ion-specific reaction dynamics fundamentally control laser-induced cavitation in aqueous environments.

Figures (8)

• Figure 1: Experimental methods. (a) Schematic illustration of the preparation of the electrolyte solutions in this study via magnetic stirring. NTA microscopic snapshot shows trace levels of nanoscale contaminants in the formulated aqueous solution. (b) Schematic diagram of the experimental setup for pulsed laser focusing-induced optical breakdown and cavitation in electrolyte solutions.
• Figure 2: Number of cavitation bubbles $N_{c}$ formed in (a) one-component solutions and (b) mixed solutions as a function of laser pulse energy density $E$. Error bars represent the standard error of the mean. All dashed lines represent the best linear fit. (c) Time-resolved images of laser-induced cavitation in selected solutions (corresponding to (b)) under identical ionic strength $I$. The laser pulse energy density for all cases is $\sim$6.25 J/cm$^2$. All snapshots share the same scale bar with a length of 2 mm. (d) Conductivity $\sigma$ as a function of concentration $C$ for different electrolyte solutions. (e) Comparison of conductivity in four different mixed electrolyte solutions (with NaCl concentration as the horizontal axis).
• Figure 3: Schematic of laser-induced plasma and cavitation bubble formation in electrolyte solution via enhanced ionization.
• Figure 4: (a) Evolution of laser-induced cavitation bubbles in ultrapure water, HCl, NaCl, and NaOH solutions. The concentration of all electrolyte solutions was kept constant at $C = 1.0$ M. The laser pulse energy density for all cases is $\sim$6.25 $\rm{J/cm^2}$. All snapshots share the same scale bar with a length of 2 mm. (b) Temporal evolution of the total cross-sectional area of cavitation bubbles ($A_c$) in ultrapure water and electrolyte solutions. (c) Comparison of the experimental data with the modified theoretical model for $A_c$ (solid lines). The calculation starts from the moment when the bubble features its maximum volume.
• Figure 5: (a) Effect of solute concentration in different electrolyte solutions (NaCl, HCl and NaOH) on cavitation bubble nucleation. These snapshots were captured 12.6 $\mu$s after the optical breakdown. All snapshots share the same scale bar of 2 mm. (b) Threshold of optical breakdown $E_c$, (c) cavitation bubble number $N_c$, and (d) length of cavitation zone $L_c$ along the beam path as a function of electrolyte concentration $C$ in three kinds of electrolyte solutions. Error bars represent the standard error of the mean.